Fuel cells are devices that convert fluid streams containing a fuel, for example hydrogen, and an oxidizing species, for example, oxygen or air, to electricity, heat and reaction products. Such devices comprise an anode, where the fuel is provided; a cathode, where the oxidizing species is provided; and an electrolyte separating the two. The fuel and/or oxidant typically are a liquid or gaseous material. The electrolyte is an electronic insulator that separates the fuel and oxidant. It provides an ionic pathway for the ions to move between the anode where the ions are produced by reaction of the fuel, to the cathode, where they are used to produce the product. The electrons produced during formation of the ions are used in an external circuit, thus producing electricity. As used herein, fuel cells may include a single cell comprising only one anode, one cathode and an electrolyte interposed therebetween, or multiple cells assembled in a stack. In the latter case there are multiple separate anode and cathode areas wherein each anode and cathode area is separated by an electrolyte. The individual anode and cathode areas in such a stack are each fed fuel and oxidant; respectively, and may be connected in any combination of series or parallel external connections to provide power. Additional components in a single cell or in a fuel cell stack may optionally include means to distribute the reactants across the anode and cathode, including, but not limited to porous gas diffusion media and/or so-called bipolar plates, which are plates with channels to distribute the reactant. Additionally, there may optionally be means to remove heat from the cell, for example by means of separate channels in which a cooling fluid can flow.
A Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a type of fuel cell where the electrolyte is a polymer electrolyte. Other types of fuel cells include Solid Oxide Fuel Cells (SOFC), Molten Carbonate Fuel Cells (MCFC), Phosphoric Acid Fuel Cells (PAFC), etc. As with any electrochemical device that operates using fluid reactants, unique challenges exist for achieving both high performance and long operating times. In order to achieve high performance it is necessary to reduce the electrical and ionic resistance of components within the device. Recent advances in the polymer electrolyte membranes have enabled significant improvements in the power density of PEMFCs. Steady progress has been made in various other aspects including lowering Pt loading, extending membrane life, and achieving high performance at different operating conditions. However, many technical challenges are still ahead. One of them is for the membrane electrode assembly (MEA) to meet the lifetime requirements for various potential applications. These range from hundreds of hours for portable applications to 5,000 hours or longer for automotive applications to 40,000 hours or longer in stationary applications. In all cases, the membrane must not fail, and there must not be severe electrode degradation.
As is known in the art, decreasing the thickness of the polymer electrolyte membrane can reduce the membrane ionic resistance, thus increasing fuel cell power density. Within this application power density is defined as the product of the voltage and current in the external circuit divided by the geometric area of the active area in the cathode. The active area is the area in the cathode where the catalyst has access to oxidant in the cathode electrode.
However, reducing the membranes physical thickness can increase the susceptibility to damage from other device components leading to shorter cell lifetimes. Various improvements have been developed to mitigate this problem. For example, U.S. Pat. No. RE 37,307 to Bahar et al., shows that a polymer electrolyte membrane reinforced with a fully impregnated microporous membrane has advantageous mechanical properties. Although this approach is successful in improving cell performance and increasing lifetimes, even longer life would be desirable.
During normal operation of a fuel cell or stack the power density typically decreases as the operation time goes up. This decrease, described by various practitioners as voltage decay, fuel cell durability, or fuel cell stability, is not desirable because less useful work is obtained as the cell ages during use. Ultimately, the cell or stack will eventually produce so little power that it is no longer useful at all. The causes of this power loss with time are not completely understood, but are thought to occur because of various forms of degradation of the materials present in the fuel cell. For example, consider the degradation in the properties of the electrodes. Such electrode degradation mechanisms can include but are not limited to reduction of catalyst activity through reduction of effective area resulting from particle sintering or agglomeration; physical loss of the catalyst, catalyst support or ionically conducting component in the electrode; degradation of the interfaces between the electrode and adjacent components; or degradation of the interfaces within the multiple phases present within the electrode.
One of these mechanisms, the physical loss of catalyst from the electrode, is particularly relevant to this application. Under certain conditions in a fuel cell, Pt metal is thermodynamically unstable and should corrode in the electrode. [See Van Muylder, J., DeZoubov, N., & Pourbaix, M.; Platinum, in Atlas of Electrochemical Equilibria, 1974 edn, M. Pourbaix, ed., National Association of Corrosion Engineers, Houston, Tex., pp. 378-383]. The extent of corrosion will depend on a number of factors, including but not limited to the local conditions near the Pt in the cell, the kinetics of the dissolution reaction, and the temperature. Although the fact that Pt may corrode in the fuel cell has been understood in the art for some time, the extent to which it does so, and methods and techniques to mitigate such corrosion have not been previously delineated.
Another critical variable in the operation of fuel cells is the temperature at which the cell is operated. Although this varies by the type of system, for PEMFCs, the operating temperature is less than about 150 degrees Celsius. PEMFCs are more typically operated between 40 and 80 degrees Celsius because in that temperature range the power output is acceptably high, and the voltage decay with time is acceptably low. At higher temperature, decay rates tend to increase, and cell durability thereby decreases. It would be highly desirable to operate at higher temperatures, for example between about 90 and 150 degrees Celsius, though. By so doing the effects of potential poisons, for example carbon monoxide, would be reduced. Furthermore, above 100 degrees Celsius, flooding and other deleterious effects of water are less of an issue. Yet, with current materials and operating conditions lifetimes are unacceptably short at these higher temperatures.
Yet another factor that is currently limiting the broad acceptance of fuel cells is their cost. In part, this is due to the presence of relatively large amounts of precious metal catalysts, i.e., Pt, in the electrodes. Historically the concentration of Pt in the electrodes in state-of-the-art fuel cells has decreased from a ˜5-10 mg/cm2 Pt loading 30 years ago to ˜1 mg/cm2 today. Yet, to meet aggressive cost targets for high volume transportation applications, loading levels will need to decrease by as much as an additional order of magnitude. Such low loadings will require very low electrode degradation during cell operation because there will be no “reserve” Pt present in the electrode as there is with today's loading levels. For example, in a cell with an initial loading of 0.4 mg/cm2 Pt in an electrode, if 0.1 mg/cm2 becomes inactive or is lost because of degradation during fuel cell operation there will still be 0.3 mg/cm2 “active” Pt catalyst available. On the other hand, the same amount of Pt activity loss in a cell that began with ˜0.1 mg/cm2 would lead to complete cell failure because there will be little or no Pt available to catalyze the reactions. Thus, it becomes increasingly important to reduce or eliminate electrode degradation mechanisms in fuel cells that render Pt catalytically inactive.
Although there have been many improvements to fuel cells in an effort to improve life of fuel cells, most have focused on using improved materials. Very few have focused on specific operational methods or means of operating a fuel cell that would act to maximize lifetimes or durability of a fuel cell.